A flow rate control valve device is so far known which is arranged as shown in FIG. 1 such that a main poppet 3 is provided to connect or disconnect a main port 1 for flow rate controlling purposes with or from a drain port 2, and the main poppet 3 is biased by a spring 4 to its cut-off position and has a drain hole 5 formed therein and adapted to communicate the main port 1 with the low pressure drain port 2, the drain hole 2, the drain hole 5 being arranged to be opened and closed freely by a stem 7 adapted to be moved by a pilot piston 6, the pilot piston 6 being moved by a pilot pressure within a pilot pressure chamber 8 so as to move the stem 7 to open the drain hole 5 to thereby cause fluid flow from the main port 1 into the drain port 2, and the main poppet 3 being moved through the action of a high pressure fluid exerted on a shoulder 3 of the main poppet 3 against the resilient force of the spring 4 so as to discharge fluid under pressure from the main port 1 into the drain port 2 thereby controlling the fluid flow through the main port 1.
Such a flow rate control valve is arranged such that the pilot piston 6 is displaced by the pilot pressure so as to cause a displacement of the main poppet 3 to thereby control the flow rate of fluid under pressure, however, it is disadvantageous in that its response is poor because there is a time difference between the commencement of the inflow of fluid under pressure and the occurrence of a change in the flow rate of fluid under pressure flowing therethrough.
Further, this flow rate control valve is arranged such that the stem 7 is moved by a pilot pressure supplied into a back pressure chamber 7a formed in the rear part thereof so as to control the fluid pressure within the spring chamber of the main poppet 3 to thereby control the rate of pressurized fluid flow therethrough. Therefore, provision of a pilot pressure supplying mechanism is required so that the system per se becomes complicated in structure, and also not only errors in supply of the pilot pressure bring about changes in the stroke of the stem 7, but also leakage of fluid under pressure through a sliding portion 9 of the stem 7 into the pilot pressure chamber 8 is liable to occur, thus rendering it impossible to control the flow rate of fluid under pressure at a high accuracy.